The accessibility of the Canvas tool itself is fairly good and is steadily improving. This document addresses how to make the content you add to Canvas accessible.
Content added with the editor
Check a page for accessibility
As you go along using the editor, check your work with Canvas' Accessibility Checker. The Accessibility Checker's button is in the toolbar and looks like Da Vinci's Vitruvian Man. This is a simple user-friendly way of making your pages accessible as you go about creating them. Canvas has instructions for using the Accessibility Checker.
Check a full course site
If you are checking the content of a whole course site, you may want to consider using UDOIT. This Canvas add-in tool will scan the whole site, provide you with a report, and guide you through fixing things. For guidance, see this video tutorial on UDOIT (19:06).
Content added from your computer
Please take a look at the guide on how to create accessible documents for the most common formats. It also has instructions on how to check the accessibility of pre-existing documents.